This invention relates to stable suspensions of water-soluble polymers, their manufacture and their use for improving enhanced oil recovery.
The world energy crisis has made it essential to recover as much oil as possible from oil fields.
One of the most widely used methods of enhanced oil recovery consists of flooding the oil field wih injected saline water to displace the oil from the porous rock.
The efficiency of this method is, however, very often limited by the viscosity difference between oil and water, which difference causes water to channel directly from the injection well into the production well, instead of spreading through the whole deposit.
It is conventional to reduce this viscosity difference by thickening the injection water with water-soluble polymers, usually with an acrylamide polymer or co-polymer or with a polysaccharide.
The storage and processing of these polymers in the field has raised a number of practical problems.
As a matter of fact, the polymer powder readily absorbs water from the air, resulting in swelling and the formation of agglomerates and/or microgels. These agglomerates tend to stick on the walls, and thus inhibit the operation of the mixing devices, particular the feed screw.
The microgels, on the other hand, do not dissolve easily in water and, once injected into the field, they tend to obstruct the field by clogging the pores of the oil-containing rocks.
Still further, the polymer powder when spread on the ground and into the air, makes the ground thick and slippery and the atmosphere hardly breathable, thus raising environmental and safety problems.
It has been proposed, in the prior art, to obviate these various drawbacks by using certain techniques for dispersing solid particles of water-soluble polymers in a liquid which is a non-solvent for the polymer.
It has thus been taught to admix the polyacrylamide with glycerin (for example, in U.S. Pat. No. 3,839,202), with polyethylene glycol (for example, in U.S. Pat. No. 3,402,137 or British Pat. No. 1,387,367), with ethylene glycol in the presence of an emulsifying agent (for example, in U.S. Pat. No. 3,657,182) or with an organic water-immiscible liquid to which water is added in order to make the polymer swell (for example, in the U.S. Pat. No. 3,282,874).
However the main difficulty, when preparing polymer suspensions according to the above methods of the prior art, lies in the instability of the suspensions during storage, resulting in their decantation or thickening due to the progressive swelling of the polymer in the presence of one of the ingredients; this instability in the course of time results in the formation of compositions which are no longer uniform and which are thickened. Their processing and their use become therefore more difficult and hazardous.
Compositions of water-soluble polymers and particularly ethylene oxide polymers, which dissolve easily in water, have been described, for example, in the European Patent Application No. 0 002 368. When fluid compositions are obtained by dispersing a water-soluble polymer in an organic liquid which is insoluble in water, in the presence of a non-ionic emulsifying agent, there is observed a separation of the compositions when stored at room temperature. It is the reason why it has also been proposed to add to the composition a thickening agent such as finely dispersed silica, asbestos or soaps such as aluminum stearate. However, these compositions are not adapted to the further preparation of aqueous solutions of polymers to be used in the enhanced oil recovery. As a matter of fact, it has been observed that the thickening agents of the prior art result in secondary reactions of cross-linking or degradation in the presence of the water-soluble polymers used in enhanced oil recovery, more particularly acrylamide polymers or co-polymers. These secondary reactions result in the formation of microgels which clog the porous media where they induce a viscosity decrease of the aqueous solutions and consequently a loss of efficiency for oil recovery.
It has also been proposed in the prior art to effect enhanced oil recovery with the use of aqueous emulsions of polymers, as directly prepared when manufacturing the polymer, as disclosed, for example, in the U.S. Pat. Nos. 3,637,564, 3,734,873 and 3,763,071. The addition of water, which is required in this technique, is however unfavorable to the transportation and the storage of the product in the field.